Method for s/tem sample analysis

ABSTRACT

An improved method and apparatus for S/TEM sample preparation and analysis. Preferred embodiments of the present invention provide improved methods for TEM sample creation, especially for small geometry (&lt;100 nm thick) TEM lamellae. Preferred embodiments of the present invention also provide an in-line process for S/TEM based metrology on objects such as integrated circuits or other structures fabricated on semiconductor wafer by providing methods to partially or fully automate TEM sample creation, to make the process of creating and analyzing TEM samples less labor intensive, and to increase throughput and reproducibility of TEM analysis.

The present application is a continuation of U.S. application Ser. No.14/546,244, filed Nov. 18, 2014, which is a continuation of U.S.application Ser. No. 13/777,018, filed Feb. 26, 2013, which is acontinuation of Ser. No. 12/446,387, filed Sep. 16, 2009, which is a 371National Phase filing of PCT Application PCT/US2007/082166, filed Oct.22, 2007, which claims priority from U.S. Prov. Pat. App. No.60/853,183, filed Oct. 20, 2006, all of which are hereby incorporated byreference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to preparation of samples and methods ofanalysis for transmission electron microscopes and scanning transmissionelectron microscopes.

BACKGROUND OF THE INVENTION

Semiconductor manufacturing, such as the fabrication of integratedcircuits, typically entails the use of photolithography. A semiconductorsubstrate on which circuits are being formed, usually a silicon wafer,is coated with a material, such as a photoresist, that changessolubility when exposed to radiation. A lithography tool, such as a maskor reticle, positioned between the radiation source and thesemiconductor substrate casts a shadow to control which areas of thesubstrate are exposed to the radiation. After the exposure, thephotoresist is removed from either the exposed or the unexposed areas,leaving a patterned layer of photoresist on the wafer that protectsparts of the wafer during a subsequent etching or diffusion process.

The photolithography process allows multiple integrated circuit devicesor electromechanical devices, often referred to as “chips,” to be formedon each wafer. The wafer is then cut up into individual dies, eachincluding a single integrated circuit device or electromechanicaldevice. Ultimately, these dies are subjected to additional operationsand packaged into individual integrated circuit chips orelectromechanical devices.

During the manufacturing process, variations in exposure and focusrequire that the patterns developed by lithographic processes becontinually monitored or measured to determine if the dimensions of thepatterns are within acceptable ranges. The importance of suchmonitoring, often referred to as process control, increases considerablyas pattern sizes become smaller, especially as minimum feature sizesapproach the limits of resolution available by the lithographic process.In order to achieve ever-higher device density, smaller and smallerfeature sizes are required. This may include the width and spacing ofinterconnecting lines, spacing and diameter of contact holes, and thesurface geometry such as corners and edges of various features. Featureson the wafer are three-dimensional structures and a completecharacterization must describe not just a surface dimension, such as thetop width of a line or trench, but a complete three-dimensional profileof the feature. Process engineers must be able to accurately measure thecritical dimensions (CD) of such surface features to fine tune thefabrication process and assure a desired device geometry is obtained.

Typically, CD measurements are made using instruments such as a scanningelectron microscope (SEM). In a scanning electron microscope (SEM), aprimary electron beam is focused to a fine spot that scans the surfaceto be observed. Secondary electrons are emitted from the surface as itis impacted by the primary beam. The secondary electrons are detected,and an image is formed, with the brightness at each point of the imagebeing determined by the number of secondary electrons detected when thebeam impacts a corresponding spot on the surface. As features continueto get smaller and smaller, however, there comes a point where thefeatures to be measured are too small for the resolution provided by anordinary SEM.

Transmission electron microscopes (TEMs) allow observers to seeextremely small features, on the order of nanometers. In contrast SEMs,which only image the surface of a material, TEM also allows analysis ofthe internal structure of a sample. In a TEM, a broad beam impacts thesample and electrons that are transmitted through the sample are focusedto form an image of the sample. The sample must be sufficiently thin toallow many of the electrons in the primary beam to travel though thesample and exit on the opposite site. Samples, also referred to aslamellae, are typically less than 100 nm thick.

In a scanning transmission electron microscope (STEM), a primaryelectron beam is focused to a fine spot, and the spot is scanned acrossthe sample surface. Electrons that are transmitted through the workpiece are collected by an electron detector on the far side of thesample, and the intensity of each point on the image corresponds to thenumber of electrons collected as the primary beam impacts acorresponding point on the surface.

Because a sample must be very thin for viewing with transmissionelectron microscopy (whether TEM or STEM), preparation of the sample canbe delicate, time-consuming work. The term “TEM” as used herein refersto a TEM or an STEM and references to preparing a sample for a TEM areto be understood to also include preparing a sample for viewing on anSTEM. The term “S/TEM” as used herein also refers to both TEM and STEM.

Several techniques are known for preparing TEM specimens. Thesetechniques may involve cleaving, chemical polishing, mechanicalpolishing, or broad beam low energy ion milling, or combining one ormore of the above. The disadvantage to these techniques is that they arenot site-specific and often require that the starting material besectioned into smaller and smaller pieces, thereby destroying much ofthe original sample.

Other techniques generally referred to as “lift-out” techniques usefocused ion beams to cut the sample from a substrate or bulk samplewithout destroying or damaging surrounding parts of the substrate. Suchtechniques are useful in analyzing the results of processes used in thefabrication of integrated circuits, as well as materials general to thephysical or biological sciences. These techniques can be used to analyzesamples in any orientation (e.g., either in cross-section or in planview). Some techniques extract a sample sufficiently thin for usedirectly in a TEM; other techniques extract a “chunk” or large samplethat requires additional thinning before observation. In addition, these“lift-out” specimens may also be directly analyzed by other analyticaltools, other than TEM. Techniques where the sample is extracted from thesubstrate within the FIB system vacuum chamber are commonly referred toas “in-situ” techniques; sample removal outside the vacuum chamber (aswhen the entire wafer is transferred to another tool for sample removal)are call “ex-situ” techniques.

Samples which are sufficiently thinned prior to extraction are oftentransferred to and mounted on a metallic grid covered with a thinelectron transparent film for viewing. FIG. 1A shows a sample mountedonto a prior art TEM grid 10. A typical TEM grid 10 is made of copper,nickel, or gold. Although dimensions can vary, a typical grid mighthave, for example, a diameter of 3.05 mm and have a middle portion 12consisting of cells 14 of size 90×90 μm² and bars 13 with a width of 35μm. The electrons in an impinging electron beam will be able to passthrough the cells 14, but will be blocked by the bars 13. The middleportion 12 is surrounded by an edge portion 16. The width of the edgeportion is 0.225 mm. The edge portion 16 has no cells, with theexception of the orientation mark 18. The thickness 15 of the thinelectron transparent support film is uniform across the entire samplecarrier, with a value of approximately 20 nm. TEM specimens to beanalyzed are placed or mounted within cells 14.

For example, in one commonly used ex-situ sample preparation technique,a protective layer 22 of a material such as tungsten is deposited overthe area of interest on a sample surface 21 as shown in FIG. 2 usingelectron beam or ion beam deposition. Next, as shown in FIGS. 3-4, afocused ion beam using a high beam current with a correspondingly largebeam size is used to mill large amounts of material away from the frontand back portion of the region of interest. The remaining materialbetween the two milled rectangles 24 and 25 forming a thin verticalsample section 20 that includes an area of interest. The trench 25milled on the back side of the region of interest is smaller than thefront trench 24. The smaller back trench is primarily to save time, butthe smaller trench also prevents the finished sample from falling overflat into larger milled trenches which may make it difficult to removethe specimen during the micromanipulation operation.

As shown in FIG. 5, once the specimen reaches the desired thickness, thestage is tilted and a U-shaped cut 26 is made at an angle partiallyalong the perimeter of the sample section 20, leaving the sample hangingby tabs 28 at either side at the top of the sample. The small tabs 28allow the least amount of material to be milled free after the sample iscompletely FIB polished, reducing the possibility of redepositionartifacts accumulating on the thin specimen. The sample section is thenfurther thinned using progressively finer beam sizes. Finally, the tabs28 are cut to completely free the thinned lamella 27. Once the finaltabs of material are cut free lamella 27 may be observed to move or fallover slightly in the trench. A completed and separated lamella 27 isshown in FIG. 6.

The wafer containing the completed lamella 27 is then removed from theFIB and placed under an optical microscope equipped with amicromanipulator. A probe attached to the micromanipulator is positionedover the lamella and carefully lowered to contact it. Electrostaticforces will attract lamella 27 to the probe tip 29 as shown in FIG. 7.The tip 29 with attached lamella is then typically moved to a TEM grid10 as shown in FIG. 8 and lowered until lamella is placed on the grid inone of the cells 14 between bars 13.

Whichever method is used, the preparation of sample for TEM analysis isdifficult and time consuming. Many of the steps involved in TEM samplepreparation and analysis must be performed using instruments operatedmanually. For this reason, successful TEM sample preparation generallyrequires the use of highly trained and experienced operators andtechnicians. Even then, it is very difficult to meet any reasonablestandards of reproducibility and throughput.

Use of FIB methods in sample preparation has reduced the time requiredto prepare samples for TEM analysis down to only a few hours. However,CD metrology often requires multiple samples from different locations ona wafer to sufficiently characterize and qualify a specific process. Insome circumstances, for example, it will be desirable to analyze from 15to 50 TEM samples from a given wafer. When so many samples must beextracted and measured, using known methods the total time to processthe samples from one wafer can be days or even weeks. Even though theinformation that can be discovered by TEM analysis can be very valuable,the entire process of creating and measuring TEM samples hashistorically been so labor intensive and time consuming that it has notbeen practical to use this type of analysis for manufacturing processcontrol.

What is needed is a method to more completely automate the process ofTEM sample creation, extraction, and measurement and to increasethroughput and reproducibility so that TEM measurement can beincorporated into integrated or in situ metrology for process control.

SUMMARY OF THE INVENTION

An object of the invention, therefore, is to provide an improved methodfor TEM sample analysis. Preferred embodiments of the present inventionprovide improved methods for TEM sample creation, extraction,measurement, and data handling, especially for small geometry (<100 nmthick) TEM lamella. Some preferred embodiments of the present inventionprovide methods to partially or fully automate TEM sample extraction andmeasurement, to make the process of creating and analyzing TEM samplesless labor intensive, and to increase throughput and reproducibility ofTEM analysis.

Another object of the invention is to reduce the time it takes toacquire data from TEM analysis of one or more sample sites so that TEMmeasurement can be incorporated into integrated or in situ metrology forprocess control. Preferred embodiments of the present invention alsoprovide an in-line process for S/TEM based metrology on objects such asintegrated circuits or other structures fabricated on semiconductorwafer.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a typical prior art TEM grid.

FIGS. 2-5 illustrate the steps in an ex-situ sample preparationtechnique according to the prior art.

FIG. 6 is a micrograph of a completed and separated lamella according tothe prior art.

FIGS. 7-8 illustrate the transfer of a lamella using a probe andelectrostatic attraction according to the prior art.

FIG. 9 illustrates automated S/TEM sample management according to thepresent invention.

FIG. 10 is a flowchart showing the steps of creating, processing, andmeasuring one or more TEM samples according to a preferred embodiment ofthe present invention.

FIG. 11 is a flowchart showing the steps of creating one or morelamellae according to a preferred embodiment of the present invention.

FIG. 12 shows a lamella site according to the process of FIG. 11 afterhigh precision fiducials have been milled and a protective layerdeposited over the lamella location.

FIG. 13 shows a lamella site according to the process of FIG. 11 afterlow precision fiducials have been milled.

FIG. 14 shows a lamella site according to the process of FIG. 11 afterbulk milling has been completed.

FIG. 15 shows a high resolution micrograph of a lamella sample accordingto the present invention after bulk milling has been completed.

FIG. 16 shows a lamella created according to the process of FIG. 11.

FIG. 17 shows a lamella created according to the process of FIG. 11.

FIG. 18 shows a high resolution micrograph of a lamella according to thepresent invention.

FIG. 19A shows a graphical representation of a dual beam system whereone beam is used to thin the lamella while the other beam images thelamella to endpoint milling.

FIG. 19B shows a graphical representation of a single beam system wherethe sample must be rotated to allow one beam to mill and image forendpointing.

FIG. 19C shows a lamella site during the milling process which could beimaged and the image processed according to the present invention toendpoint milling.

FIG. 20 shows a lamella suitable for extraction with an ex-situ sampleextraction device according to the present invention.

FIG. 21 shows an ex-situ lamella extraction device according to thepresent invention.

FIG. 22 is a flowchart showing the steps in extracting a sample using anex-situ sample extraction device according to the present invention.

FIG. 23 is an example of a ray diagram showing the possible path of abeam of light when using top down illumination.

FIG. 24 is an example of a ray diagram showing the possible path of abeam of light when using oblique illumination.

FIG. 25 shows a beveled probe according to the present invention.

FIGS. 26A-26B illustrate lowering a probe tip into contact with a sampleto be extracted according to the present invention.

FIGS. 27-28 illustrate moving a probe tip into contact with a sample tobe extracted according to the present invention.

FIGS. 29-32 illustrate steps in the transfer of an extracted sample to aTEM grid according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention provide an in-lineprocess for S/TEM based metrology on objects such as integrated circuitsor other structures fabricated on semiconductor wafer. The process maybe partially or fully automated and can be utilized in a waferfabrication facility to provide rapid feedback to process engineers totroubleshoot or improve processes.

Other embodiments of the present invention provide improved methods forlamella creation, extraction, and measurement. A preferred embodimentcan create S/TEM samples with a thickness in the 50-100 nm range for thepurposes of S/TEM metrology with minimal site-to-site variation. Theprocess can produce a 10 μm wide×˜5 μm deep×˜500 nm thick lamella with afinal-thinned window of 3 μm×3 μm at the targeted final thickness(50-100 nm). The entire process is preferably fully automated andproduces a lamella in roughly 18 minutes, with a site-to-site 3-sigmafinal lamella thickness variation of roughly 20 nm.

A preferred method or apparatus of the present invention has many novelaspects, and because the invention can be embodied in different methodsor apparatuses for different purposes, not every aspect need be presentin every embodiment. Moreover, many of the aspects of the describedembodiments may be separately patentable.

Sample Management

FIG. 9 illustrates automated S/TEM sample management according to thepresent invention. In the preferred embodiment of FIG. 9, TEM samplesare processed by a cluster of different processing tools having thecapability of sequentially processing samples (e.g., lamellae extractedfrom semiconductor wafers). The S/TEM sample management tool suite 100generally includes a Process Controller 110 and a Fab Host computer 112operably connected to (or integrated with) a FIB system 114, a lamellaextraction tool 116, and a S/TEM system 118. In a preferred embodiment,FIB system 114 comprises a dual beam FIB/SEM system such as theCertus/CLM available from FEI Company of Hillsboro, Oreg., the assigneeof the present invention; and S/TEM system 118 comprises a system suchas a Tecnai G2 S/TEM also available from FEI Company. In the preferredembodiment of FIG. 9, each processing tool is operably connected to (orintegrated with) a computer station 120, which uses software 122 forimplementing TEM sample creation and processing. Any suitable software(conventional and/or self-generated) applications, modules, andcomponents may be used for implementing software. For example, in theembodiment of FIG. 9, the automated S/TEM sample management isimplemented using IC3D™ software for automated machine control andmetrology, which is also available from FEI Company.

With reference to FIG. 9, in one embodiment, Process Controller 110 isshown connected to computer stations 120 through network 130. Network130 may be any suitable network configuration, such as a virtual privatenetwork (“VPN”), local area network (“LAN”), wide area network (“WAN”)or any combination thereof. Similarly, computers for performing processcontroller 110 functions, Fab Host 112 functions, controller 124function, computer station 120 functions, or data storage 126 functionsmay be any suitable computing devices such as desktop computers, laptopcomputers, PDAs, server systems, mainframes, processing systems madefrom discrete components, and/or combinations of one or more of thesame. They can execute conventional operating systems such as Windows™,Unix™ Linux™, Solaris™ and/or customized, job-specific operatingsystems. The depicted devices can be implemented with any suitablecombination of conventional (albeit possibly modified) equipment, and inmany system embodiments, may not even be included. (For example, thenetwork 130 may not be utilized.)

FIG. 10 is a flowchart showing the steps of creating, processing, andmeasuring one or more TEM samples according to a preferred embodiment ofthe present invention. In step 201, at the front end of the CD-S/TEMmetrology process, Fab Host 112 schedules one or more specifiedprocesses to be executed on a particular wafer. The processes can beinput by an operator or selected from a menu of available recipes.Alternatively, processes or recipes can be scheduled by ProcessController 110. Each processing tool is coupled to a controller 124,which receives instructions from the Fab Host as well as waferidentification information. Each controller 124 also interacts withsoftware 122, such as the IC3D™ software for automated machine controland metrology.

Wafers 101 are preferably uniquely identified by a wafer ID number orother designation. Optionally, the wafer ID information can be etchedinto the surface of the wafer. The specified processes can include, forexample, instructions or recipes for locating desired lamella sites,imaging lamella sites using the SEM or FIB, milling fiducial marks atspecified locations based upon image processing and metrology, millinglamellae at precise locations on the wafer surface, and analyzinglamella samples using S/TEM analysis. In most cases, all the specifiedprocesses are preferably performed automatically, with little or no userintervention. The metrology process can be controlled by matching aparticular recipe to be run with the particular wafer ID.

In step 202, one or more wafers 101 are then diverted from theproduction line 128. Wafers are preferably transferred by way of amulti-wafer carrier and auto-loading robot, as in well known in the art,although wafers can also be transferred manually. After a carrier hasbeen transferred to the FIB system, in step 204, the wafer residing in aspecified slot of the carrier is loaded into FIB system 114, such as aCertus Dual Beam System, to mill TEM lamellae at desired locations.Optionally, in step 206, an OCR device can read the ID etched into thewafer surface to confirm that the correct wafer will be processed.

Once wafer 101 is loaded into the FIB device 114, in step 208, the sitefor each desired lamella is located. The FIB device can use x-ycoordinates provided by the FAB host 112 to roughly locate lamellasites. In step 210, the FIB navigates to a desired lamella site.Optionally, in step 212, a protective layer is deposited over the site.Then in step 214, one or more fiducial marks are milled nearby.Alternatively, one or more of the fiducials could be located on thelamella to be removed. Also, in some embodiments, the fiducial markscould be created by known techniques other than ion beam milling, forexample by SEM or FIB deposition or by SEM milling. Preferably,high-precision fiducial marks are employed and the exact locations foreach fiducial mark and each lamella can be determined by patternrecognition based upon automated machine-vision based metrology andimage recognition, as discussed in greater detail below and inco-pending PCT App. No. PCT/US07/82159, filed on Oct. 22, 2007, which ishereby incorporated by reference. If there are other lamella sites onthe wafer, in step 216, the process returns to step 210 and repeatssteps 210 to 216 at each desired lamella site.

Once all lamella sites have been located and the desired fiducialsmilled, in step 218, the FIB navigates to an unmilled lamella site. Instep 220, a lamella is created at each desired location using theprocess described below with reference to FIG. 11. After all desiredlamellae have been milled, the samples are extracted from the wafer 101and loaded onto a TEM grid 102. Lamellae extraction can be conductedaccording to a number of known methods, either in-situ or ex-situ. In apreferred embodiment, once the lamellae have been milled, in step 224,the entire wafer is transferred to a separate lamella extraction tool116 such as an Ex-Situ Plucker (“ESP”), discussed below. Wafers arepreferably transferred by way of a multi-wafer carrier and auto-loadingrobot, as in well known in the art, although wafers can also betransferred manually. In step 226, a wafer site list (including, forexample, Site IDs, list of the x-y coordinates for each lamellalocation, Wafer ID, Lot ID, and Carrier Slot Number) is retrieved fromthe FIB system 114 by the lamella extraction tool 116. Alternatively,the wafer site list could be provided by the FAB Host or other processcontroller. Skilled persons will appreciate that whenever information orinstructions are passed from one device to another as discussed herein,the transfer can be implemented using a number of well-known methods,including direct transfer (tool to tool) or through other devices orcontrollers (such as through a process controller).

In step 228, ESP 116 uses a mechanical stage to navigate to the site ofan unextracted lamella. In step 230, the lamella is extracted, forexample, using a mechanical/electrostatic/pressure manipulator andplaced onto a TEM grid in step 231. A new TEM grid will preferably beused to mount lamellae from all sites at specified locations on the gridso that TEM data can be properly mapped to the appropriate waferlocation. If there is another lamella to be extracted, in step 232, theprocess returns to step 228 and repeats steps 228 to 232 for eachunextracted lamella.

Once all the lamellae have been extracted and the TEM grid 102populated, in step 234, the wafer 101 and grid 102 are unloaded from thelamellae extraction tool 116, and wafer 101 is then returned to theproduction line. Again, wafers are preferably transferred by way of amulti-wafer carrier and auto-loading robot, as in well known in the art,although wafers can also be transferred manually. In step 236, thepopulated grid 102 is placed in a cartridge 104 uniquely marked by thegrid identification number. In step 238, a lamella site list is importedfrom the lamella extraction tool. The lamella site list preferablycontains, among other data, the grid coordinates for each placedlamella, the corresponding Site IDs, the Wafer ID, and the Lot ID.

In step 240, the populated grid 102 is then loaded into a TEM/STEMsystem. In step 242, the samples are imaged by the S/TEM. Prior to S/TEMimaging, automatic orientation routines can be used to ensure that eachlamella is properly oriented for imaging. A preferred auto-orientationroutine can use pattern recognition powered via IC3D along withautomated stage moves and beam-shifts to rotate and center each lamellaerelative to the electron beam. Other routines can be used to raise orlower the stage so that each sample is imaged at the proper operationalheight within the tool. A predefined region of interest on the lamellacan be used to give the framework for the automatic orientationroutines. Once that region of interest is defined it can be imaged andthe image evaluated, for example via material contrast or sharpness, todetermine the best fit for all axes of the sample orientation to ensurethat it is normal to the electron beam (Alpha and Beta orientation).

In step 244, the critical dimensions measured, and in step 246, theresults are stored in a local IC3D database. After imaging, the grid canbe discarded or placed into storage. The metrology data for each lamellacan be exported from the database to the process controller 110 in step248 and used for both upstream and downstream process control. If thereare other wafers to be processed in step 250, the process returns tostep 204 and the next wafer is loaded. The entire process repeats untilall wafers have been processed.

Lamella Creation

Current TEM lamella creation processes for FIB systems use manual inputas the primary method for locating a feature or site of interest forlamella creation. Typically, once the desired lamella location ismanually located, a fiducial or locating mark is milled nearby. BecauseFIB imaging necessarily causes some sample damage, a protective layer isdeposited over the desired lamella location before imaging and/ormilling. The protective layer makes it harder to see features on thesubstrate so a fiducial mark is typically milled into the protectivelayer to help orient the beam and locate the proper place for a cut.This fiducial is used in subsequent processing as a locating mark. Imagerecognition keyed to this fiducial is then used to find the locationsfor subsequent milling of the lamella. In order to mill the fiducial, alocation near the desired lamella site is typically selected manually,and the desired fiducial pattern is then automatically milled at thatlocation.

This method of manually identifying the lamella site and then manuallyselecting the fiducial location does not provide a high degree ofprecision or accuracy. As a result, known automatic lamella millingroutines are limited to rough milling of lamellae which areapproximately 500 nm thick. Further thinning is typically manuallycontrolled in order to achieve the desired lamella thicknesses of 100 nmor less.

FIG. 11 is a flowchart showing the steps of creating one or morelamellae according to a preferred embodiment of the present invention.In this embodiment, machine-vision based metrology and imagerecognition, high-precision fiducial marks, and automatic fiducialplacement are used to significantly improve lamella placement accuracyand precision. Various steps in the process are shown in FIGS. 5 through11.

First, in step 301, a wafer is loaded into a FIB system, such as aCertus Dual Beam System, commercially available from FEI Company ofHillsboro, Oreg., the assignee of the present invention. In step 302,lamella sites on the wafer surface are located automatically using imagerecognition software. Suitable image recognition software is available,for example, from Cognex Corporation of Natick, Mass. Image recognitionsoftware can be “trained” to locate the desired lamella locations byusing sample images of similar features or by using geometricinformation from CAD data. Automated FIB or SEM metrology can also beused to identify or help identify the lamella site. Metrology mayconsist of image-based pattern recognition, edge finding, ADR,center-of-mass calculations, blobs, etc.

In optional step 304, the lamella site is given a protective 5 kV FIBtungsten deposition 15 μm wide by 3 μm tall for 1:20. This providessufficient tungsten on the site surface to prevent damage during the 30kV FIB site alignment and deposition steps. This protective layer may bedirectly placed if the 5 kV180 pA FIB aperture to SEM coincidence isless than 4 μm, otherwise a process of site alignment may be used torefine placement of this deposition.

In step 306, the precise locations of any desired fiducial marks withrespect to each desired lamella location are specified. For example,using a FIB or SEM to image a sample location, a fiducial location couldbe specified by an operator using a mouse to drag a virtual box aroundthe desired fiducial location. Automated metrology software could thenprecisely measure the location of the fiducial with respect toidentifiable features at the sample location (for example 15 nm from theright edge of the feature). When each lamella site is located, afiducial can then be automatically milled at each lamella site at theprecise location specified so that the spatial relationship between eachfiducial and each lamella location will be identical. A fiduciallocation could also be specified using CAD data to specify the locationof the fiducial with respect to a particular structure on the wafersurface.

In a preferred embodiment, precise fiducial placement is accomplishedthrough the use of the IC3D™ software's vision tools. A specifiedpattern can be located by image recognition software and used to locatea target structure. A series of calipers—a pattern recognition tool thatlocates edges—are then used to find the edges of the target structureand to precisely center the fine fiducials around the target structure.Extensive use of IC3D's shape linking capabilities allows robustplacement of site fiducials based on direct measurement of each site.

Preferably, a combination of high precision (fine) fiducials and lowprecision (bulk) fiducials are used to optimize lamella placementprecision and accuracy. Currently, fiducials used for lamella locationand milling consist only of low-precision features such as an “X” formedby the intersection of two milled lines. At the resolutions necessaryfor adequate lamella production, however, each milled line will beseveral nanometers wide. Edge detection software must be used todetermine the centerline of each milled line and then the intersectionof the two mathematically determined centerlines used to determine aparticular reference point. There is typically too much error in thistype of determination to use the fiducial to accurately determine alamella location within the margin of error needed for manysmall-geometry lamella applications.

In a preferred embodiment, a combination of typical low-precisionfiducial marks and higher precision marks are used. High-precisionfiducials, such as the rectangles 406 shown in FIG. 12 allow the lamellalocation to be much more accurately determined. The rectangularfiducials 406 shown in FIG. 12 are located at either end of the desiredlamella location 427. High-precision fiducial are smaller than thelow-precision fiducials discussed below. For this reason, thehigh-precision fiducials are not identifiable with the large FIB beamsused for bulk milling, and are only used for final placement of thelamella with smaller FIB beams. The rectangular fiducials in FIG. 12 arelocated using image analysis to determine the Y position of their topand bottom edges. This results accurate positioning even when thefiducial is damaged during FIB imaging. Edge detection software only hasto identify the top and bottom edges to precisely locate the top andbottom edges of the lamella. Pattern recognition for these rectangularfiducials is based on a two-measurement strategy—the top and bottomedges of the fiducial are measured. Once the edge positions are located,a central line or axis can be determined which is parallel to the topand bottom edges of the lamella. As the substrate 403 is imaged with theFIB, the top surface is progressively sputtered away. The high precisionfiducial described above is very tolerant of this FIB damage becauseboth measured edges will be altered at nearly the same rate, so theoverall error in lamella placement will be very low.

Low-precision fiducials, such as the large circles 404 in FIG. 13, canbe used for gross-structure pattern recognition, such as quickly findingthe approximate lamella location and performing the bulk milling.Suitable low-precision fiducials can be easily identified when thesample is imaged with a low resolution (higher beam size) ion beamsuitable for rapid bulk material removal. Multiple fiducials andcombinations of low and high precision fiducials and different fiducialshapes (as shown in FIG. 13) can be used for even more accurateorientation.

Once the fiducial locations have been determined, in step 308, highprecision fiducials are milled at the desired locations. As shown inFIG. 12, a small rectangular feature 406 is milled at each end of thelamella site (which is indicated by dashed line 407) with the 1 nA 30 kVFIB for vertical placement of the lamella during the final thinningprocess. In a preferred embodiment, a suitable fiducial pattern willallow the final lamella placement to be accurate within 10 nm. In someembodiments, the size and shape of the fiducial can be varied dependingon the size, width, or location of the desired lamella.

In step 310, after the high precision fiducials have been milled, a bulkprotective layer 408 composed of, for example, tungsten or platinum isdeposited over the lamella site to protect the sample from damage duringthe milling process. FIG. 13 shows a lamella site 402 with a protectivelayer 408 deposited over the desired lamella location on a wafer surface403. For some samples where information is required very close to thesurface, it may be useful to deposit the protective layer using a lowenergy FIB (˜5 keV) to perform the deposition. The high precisionfiducials 406 are also preferably lightly backfilled with the protectivematerial to protect them during future processing.

In step 312, after the bulk protective deposition, large circularfiducials 404 as shown in FIG. 13 are milled around the fine fiducials.These low-precision fiducials are used for gross-structure patternrecognition, such as quickly re-finding the approximate lamella locationand determining the location for bulk milling of the lamella. Because alarger beam size will be used for the bulk milling, a suitable lowprecision fiducial should be easily identified by pattern recognitionsoftware even in lower resolution images. The system can then readilyrelocate each desired lamella site by locating the fiducial and knowingthat the lamella site is positioned at a fixed offset from the fiducial.

If there are other lamella sites on the wafer in step 314, the FIBsystem navigates to the coordinates of the next lamella site (step 315).The process then returns to step 302 and steps 302 to 314 are repeatedfor all remaining lamella sites before the lamella milling process isstarted. Once fiducials have been milled at all lamella sites, in step316, the FIB system navigates to an unmilled lamella site. In step 318,bulk substrate milling is used to roughly shape the lamella. FIG. 14shows a lamella site after the bulk milling of step 318 has beencompleted. A larger ion beam size will be suitable for bulk materialremoval. In a preferred embodiment, each lamella will be formed by usinga FIB to cut two adjacent rectangles 424, 425 on a substrate, theremaining material between the two rectangles forming a thin verticalsample section 427 that includes an area of interest. Preferably, an ionbeam will be directed at the substrate at a substantially normal anglewith respect to the substrate surface. The beam will be scanned in arectangular area adjacent to the sample section to be extracted, thusforming a rectangular hole 424 having a predetermined depth. The milledhole should be sufficiently deep to include the feature of interest inthe extracted sample. Preferably, the milled hole is also deep enough toallow for bulk material to remain at the bottom of the thinned sample(beneath the feature of interest) to increase the mechanical rigidity ofthe sample as discussed below. The beam will be scanned in a rectangulararea 425 adjacent to the sample section to be extracted, but on theopposite side of said sample section from the first rectangular hole.The remaining material between the two rectangular holes will preferablyform a thin vertical sample section that includes the lamella to beextracted.

Low-precision fiducials 404 can be used to control the beam location forbulk milling of the lamella (using a larger beam diameter for more rapidsample removal). A typical cross-section mill pattern can be used comingin from both sides of the lamella, leaving a coarse lamellaapproximately 2 μm thick. The lamella is then further thinned toapproximately 800 nm with a cleaning cross-section mill on both sides inpreparation for the undercut step. FIG. 15 shows a high resolutionmicrograph of a lamella sample after bulk milling has been completed.

In step 320, after the fiducials and bulk mills are done, the lamellaundergoes an undercutting process. The FIB column is preferably tiltedto approximately 4.5 degrees and the lamella bottom undercut with acleaning cross-section at 1 nA. Alternatively, the sample stage could betilted. The precise location for the undercut can be located usingvision tools to locate and measure the fine fiducials. Although agreater FIB tilt could be employed (subject to hardware constraints) ashallow incidence angle undercutting provides two benefits to the TEMsample preparation process. First, the lamella face is not imaged at ahigh incidence angle, thus reducing Ga+ implantation and damage; andsecond, the undercutting process serves as an intermediate thinning stepthat has been shown to reduce the lamella thickness to a reasonablynarrow range of widths for a number of different substrates (TI SiGe, TISTI, MetroCal, IFX DTMO, Fujitsu contact). The undercut 502 and sidecuts 504 for a lamella sample 527 are shown in FIG. 16.

In step 322, the sample is then rotated 180 degrees and the processrepeated on the top edge of the lamella in order to cut the bottom free.This results in a rough lamella that is roughly 500 nm thick centeredaround the target structure.

In step 324, two cuts are made from the bottom of the lamella up to nearthe top surface in order to cut the sides of the lamella free, butleaving the lamella hanging by a tab 506 (shown in FIG. 16) on eitherside at the top of lamella. Once the final thinning of the lamella hasbeen completed, a probe can be attached to the lamella and the tabs orhinges severed so that the lamella can be extracted. Alternatively, aprobe can be used to break the lamella hinges as described below and inco-pending PCT App. No. PCT/US07/82030, filed on Oct. 20, 2007, which ishereby incorporated by reference.

In optional step 326, IC3D vision tools can be used to locate the finefiducials and remove any redeposition from the bulk milling process aswell as the protective tungsten layer deposited during the fiducialmilling process.

The lamella formed by the first two rectangular bulk-milling cuts andthe undercutting will preferably be roughly 500 nm thick. In step 328,the center portion 510 of the lamella (containing the area of interest)is thinned from both sides, preferably using a 30 pA beam at 1.2 degreesof FIB tilt with the mill pattern described below. As discussed below,the typical cleaning mill pattern commonly used for lamella millingcauses very thin lamellae (<100 nm) to bend or bow. Applicants havediscovered that using a mill pattern resulting in multiple passes of thebeam on the sample face prevents the sample from bowing. This millpattern, along with other embodiments of a method for eliminatinglamella bowing during the thinning process, is discussed in greaterdetail below.

The final thinning cuts can be placed using calipers (with imagerecognition) to find the lamella edges, with the final lamella thicknessbeing determined by an offset in the milling position from the lamellaface. For example, for each lamella to be extracted from a sample, theexact location of the lamella can be determined from the fiduciallocation. The first cut is milled at half the desired lamella thicknessaway from the center of the desired sample. Viewing the sample from thetop down, using either FIB or SEM imaging, automated metrology softwarecan then measure the edge of the first cut and the fiducial location andprecisely determine the location of the second cut. Using the locationof the high precision fiducials to precisely control beam location, thelamella can then be thinned using a finely focused FIB to a thickness of100 nm or less in a process that is also highly repeatable.

Preferably, real time pattern recognition can be used to position theFIB. A suitable FIB system providing real time pattern recognition andmetrology is the Certus 3D Dual Beam System available from FEI Company,the assignee of the present invention.

In optional step 330, low-kV cleaning is performed on the final thinnedwindow with a 180 pA 5 kV FIB at 4.5 degrees of tilt. Applicants havediscovered that a 10 second cleaning mill on each face of the lamellaproduces a significant improvement in TEM imaging conditions.

If there are other unmilled lamella sites on the wafer, in step 332, theFIB system navigates to the coordinates of the next unmilled lamellasite. The process then returns to step 316 and steps 316 to 332 arerepeated for all remaining unmilled lamella sites.

The final lamella structure produced by the method of discussed inreference to FIG. 11 is shown in FIGS. 16-18. As discussed below, acenter lamella “window” 510 can be thinned to a thickness of 100 nm orless, leaving thicker surrounding material to provide the sample withincreased mechanical strength. Preferably, the center window isapproximately 3 μm wide, 4 μm deep, and 50-70 nm thick. The thickermaterial surrounding window 510, indicated by reference numeral 512 inFIG. 15, also helps prevent the lamella from bowing during the millingprocess. The increased mechanical strength of this “windowed” lamellastructure is also very desirable when using an ex-situ lamellaextraction device as described in co-pending PCT App. No.PCT/US07/82030, filed on Oct. 20, 2007, which is incorporated byreference. FIG. 18 shows a high resolution micrograph of a lamellacreated using the process described above.

In addition to determining mill locations relative to fiducial marks asdiscussed above, the milling process can be endpointed using top downpattern recognition and metrology. In a preferred embodiment, FIBmilling is carried out in a dual beam FIB/SEM system, as shownschematically in FIG. 19A (not to scale) with vertically mounted FIBcolumn 521 used to mill substrate 403 to create lamella 427 and the SEMcolumn 522 used to image the lamella 427 so that automated metrologysoftware can determine whether the lamella 427 has been thinned to thedesired thickness. Alternatively, a dual FIB system could be used withone beam used to mill and the other used to image. As shownschematically in FIG. 19B (not to scale), a system with a single FIBcolumn 521 could also be used and the sample tilted and rotated so thatthe same beam could be used to mill and image (as is known in the priorart). Skilled persons will recognize that there is a danger of damage tothe lamella if a FIB is used to image the sample.

Referring also to FIG. 19C, after the initial bulk mill 424 is completedon one side of the lamella 427, the endpoint of the second bulk mill 425can be controlled by monitoring the width of the lamella in the samefashion that cross-sections for sub-100 nm features are measured by aCD-SEM.

Typically, to measure the width of cross-section of a structure, a SEMis used in conjunction with automatic metrology software. As theelectron beam is scanned across the exposed cross-section, whethersecondary or backscattered detection is employed, there will typicallybe a change in electron intensity at the edges of the structure. Analgorithm is used to assign an edge position based upon the contrast atthe edges of the structure and to determine the distance between thoseedges.

A preferred embodiment of the present invention makes a novelapplication of these known techniques for cross-section metrology. Thefinal lamella position and thickness would be based on a mill and imagetechnique similar to known slice and view techniques where the FIB in adual beam system is used to expose a sample cross section and the SEM isused to image the sample for automated metrology analysis. Imageprocessing tools such as pattern recognition and edge finding tools canthus be used to precisely control lamella thickness. These types ofprior art “slice and view” techniques are described, for example, inU.S. patent application Ser. No. 11/252,115 by Chitturi et al. for“Method Of Measuring Three-Dimensional Surface Roughness Of AStructure,” which is hereby incorporated by reference, and which isassigned to FEI Company, the assignee of the present invention.

Preferably, thinning would first be completed on one side of thelamella. The location of the initial milling would be controlled usingfiducial location or other metrology as discussed above. The samplewould then be imaged from the top down with either a focused ion beam orscanning electron microscope. As with a CD-SEM, when either the ion beamor the electron beam strikes the surface of substrate, secondaryelectrons and backscattered electrons are emitted. Respectively, theseelectrons will be detected by a secondary electron detector orbackscattered electron detector as is known in the art. The analogsignal produced either by secondary electron detector or backscatteredelectron detector is converted into a digital brightness values. As thebeam (either ion or electron) is scanned across the lamella surface,there will be a change in emitted electron intensity at the edges of thestructure. An algorithm is used to assign an edge position based uponthe difference in brightness values or contrast at either of the edgesof the structure and to determine the distance between those edges. Ifanalysis of the image determines that certain specified criteria are notmet (such as, for example, a minimum desired lamella/sample width) thenthe mill and image processing steps are repeated.

Lamella Extraction and Mounting

In a preferred embodiment of the present invention, once a number oflamellae have been created on a wafer, as discussed above, the lamellaecan be automatically extracted from the wafer surface and placed onto aTEM grid for S/TEM analysis. Preferably, the lamella extraction andmounting is carried out ex-situ by transferring the entire wafer fromthe FIB system to a separate lamella extraction tool such as an Ex-SituPlucker (“ESP”), described herein. Wafers are preferably transferred tothe ESP by way of a multi-wafer carrier and auto-loading robot, as inwell known in the art, although wafers can also be transferred manually.The list of all lamella sites for each wafer ID is retrieved from theFIB system 114 by the ESP. Lamella extraction tool 116 uses a mechanicalstage to navigate to each lamella site. The lamellae are extracted usinga vacuum/electrostatic manipulator and placed onto a TEM grid. Thelamella extraction process is preferably fully automated. Alternatively,the extraction process can be completely or partially controlledmanually.

FIG. 21 is a block diagram showing a preferred embodiment of ex-situsample extraction tool 610 (hereinafter “Ex-Situ Plucker” or “ESP”)according to the present invention. In a preferred embodiment, the ESPis a standalone tool for ex-situ extraction of samples. The ESPcomprises a TEM specimen extractor having a mechanical stage 612, anoptical microscope 614 with a video feed 615, and a probe 616 (alsoreferred to herein as a microprobe) for extracting the samples.Referring also to FIG. 25, probe 616 preferably comprises a cylindricalhollow tube with a flat tip beveled at approximately 45 degrees throughwhich a vacuum can be applied in order to draw the sample to the probetip. Probe 616 is oriented so that the cylindrical (longitudinal) axisof the probe is at an approximately 45 degree angle relative to thewafer (substrate) upper surface. Where the sample to be extracted has avertical sample face, this results in the probe being also oriented at a45 degree angle relative to the sample face. Probe 616 preferablycomprises a pulled 1 mm borosilicate tube with a face beveled at 45degrees. Suitable probes can be manufactured, for example, by usingstandard borosilicate micropipettes with the tips modified by using amicropipette puller such as a Narishige PC-10 to create long, thinmicro-capillaries with an outer diameter of 10 to 12 μm.

In one preferred embodiment, the ESP comprises the following componentsthat are integrated and controlled via a single control point (e.g. aControl PC) 618: a wafer holder 622 mounted on an XYZR mechanicalsubstrate stage 612, a micromanipulator system 617 including a probeholder and motors and an XYZ probe stage that can rotate a microprobeabout cylindrical (longitudinal) axis of a probe, a rotatable TEM gridholder 620, a TEM grid rotation controller 621, an optional separategrid stage (not shown) (both the wafer holder and the TEM grid holdercan be mounted on one mechanical stage), a pulled micromachinedmicropipette probe 616 with 45 degree flat tip (possibly roughened tominimize adhesion), an optional controlled environment to minimizeeffects of humidity and temperature (not shown), one or more vacuumpumps 624 or other devices for applying vacuum through the probe 616, anair pressure source 623 such as a source of compressed air, an opticalmicroscope 614 with lens 613 to image the substrate, a light opticalsystem 626 (using a fiber optic bundle 627) used to illuminate thesubstrate from an oblique angle 640 to facilitate imaging and/ormachine-based pattern recognition, a motion/contact sensor andcontroller 628, an air flow or vacuum sensor 630, and a vibrationisolation table 632.

In the preferred embodiment of FIG. 21, the ESP is operably connected to(or integrated with) a computer station 618, which uses software forimplementing sample extraction and manipulation. Computer station 618,through appropriate software, can receive the x-y coordinates for thesample to be extracted from the FIB system used to create the lamella.The location of each lamella can then be matched with a correspondingTEM grid location once the samples are extracted and transferred to theTEM grid (typically one lamella per cell). This allows for datatraceability through the entire process so that the final TEM resultscan be automatically matched back to the particular sample site on theoriginal wafer. Computer station 618 is also preferably operablyconnected to the stage controllers and micromanipulator controllers toposition the sample and grid stages and to position the microprobe.

By applying a small vacuum pressure to the lamella through themicroprobe tip, the lamella can be controlled much more accurately thanby using electrostatic force alone as in the prior art. The lamella isheld securely in place and is not as easily dropped as in the prior art.Minimizing the electrostatic attraction between the probe tip and thesample (as discussed in greater detail below) makes it much more likelythat the sample will stay precisely where it is placed rather thancontinuing to adhere to the probe tip. Even where electrostaticattraction is used to adhere the sample to the probe tip (in whole or inconjunction with vacuum pressure) the angled bevel on the microprobe,along with the ability to rotate the probe tip 180 degrees around itslong axis, allows the lamella to be placed down flat on the TEM gridfilm, which tends to maximize the attraction between the sample and theTEM grid film causing the sample to adhere to the film and stay at theposition where it is placed. This allows sample placement andorientation to be precisely controlled, thus greatly increasingpredictability of analysis and throughput (because the TEM stage doesnot need to be adjusted as often between samples).

FIG. 22 is a flowchart showing the steps in extracting multiple samplesfrom a wafer according to the present invention. These steps arepreferably carried out and controlled automatically by computer station618 via computer readable instructions, although the steps can also becompletely or partially controlled manually.

In step 701, a wafer containing milled but unextracted samples is loadedinto the ESP wafer holder 622. In a preferred embodiment, the sampleshave been created as discussed above in reference to FIGS. 2 to 6,except that the lamellae are preferably only partially separated fromthe substrate leaving a small tab of material at either end holding thelamella in place. The wafer holder 622 is mounted on an XYZR mechanicalsubstrate stage. The wafer can be aligned automatically or manuallyusing known methods.

After the wafer substrate is aligned, in step 702, the ESP can navigateto a sample site using positional data imported from the FIB system usedto create the samples. The ESP optical microscope 614 is used to imagethe substrate at the sample site. The exact sample location andorientation is determined and the probe is moved into position.Referring also to FIGS. 27-28, the sample stage is rotated so that theorientation of the probe is generally perpendicular to the lamella face(although as shown in FIGS. 21 and 26A-B the probe will typically betilted down so that the intersection of the cylindrical axis of theprobe 631 with the plane of the substrate surface forms an acute angle642). In other words, the cylindrical axis of the probe lies in a planewhich is perpendicular to the sample face. This process can be performedby an operator viewing the sample site by using manual controls to movethe probe and/or the substrate to position the probe tip in the milledcavity behind the sample. In a preferred embodiment, this process can beperformed automatically using machine-based image recognition.

Both oblique and bright field illumination should be used to facilitatesample location and grid alignment. The oblique illumination should beused to image the lamella cavity to locate the lamella to be extracted.Referring also to FIGS. 21, 26A, and 27, assuming that the lamellaitself lies in the X-Y plane 929, the illumination 672 will preferablybe directed in a plane perpendicular to the X-Y plane of the lamella 827and at an acute angle 640 relative to the substrate surface so thatlight reflecting off of the surface of the substrate 403 will not enterthe acceptance angle of the lens 613 of the optical microscope 614. Morepreferably, the illumination will be directed at an angle ofapproximately 20 degrees relative to the substrate surface.

Due to the angled walls of the lamella cavity, very poor image contrastis achieved within the cavity with top down illumination, since verylittle light enters the acceptance angle of the lens. As shown in FIGS.23 and 24, due to the multiple reflections that will occur within thecavity, the amount of signal will be highly dependent on the wall angleswithin the cavity, these may vary significantly from one application toanother. As a result, in some circumstances the illumination may need tobe directed at an angle other than the 90 degree angle described above.In a preferred embodiment, the illumination angle should be adjustable,either manually by an operator or automatically. Bright fieldillumination can be used to allow imaging of alignment marks and TEMgrids.

Unfortunately, sometimes a lamella may be missing from a cavity orpositioned improperly. It that event, it is important to be able toquickly determine whether the lamella is present. In FIG. 23, dashedline 815 represents top-down illumination with no lamella present. Asshown in FIG. 23, with top down illumination into the lamella cavity924, 925 and no lamella present, the light rays 815 will undergomultiple reflections and will not reenter the acceptance angle 806 ofthe lens (not shown). Likewise, with lamella 827 present in the cavity924, 925, light rays (shown by solid line 814) will also reflect at anangle outside acceptance angle 806. Thus, with top down illumination,the entire lamella cavity 924, 925 will appear dark (whether or not alamella 827 is present).

With oblique illumination, however, more reflected light enters thelens. As shown in FIG. 24, dashed line 825 represents obliqueillumination with no lamella present. In that case, light rays willreflect off the two cavity sidewalls and enter the lens. Thus, the lefthalf of the cavity 924 will appear brighter when the lamella is absent.Line 824 represents oblique illumination with the lamella 827 in place.In that case, reflected light will tend to escape the lens and thecavity 924, 925 will appear dark (although the top of the lamella 827may be visible.) Thus, with oblique illumination it can be readilydetermined whether a lamella is still within the cavity and whether itis at approximately the expected position. Depending upon the slope ofthe cavity walls and of the lamella face, the proper illumination anglemay be adjusted with respect to the wafer plane and with respect to anaxis perpendicular to the wafer to optimize sample location.

The oblique illumination can be supplied, for example, by way of a fiberoptic bundle mounted at an appropriate oblique angle relative to thesubstrate surface. Preferably, the illumination source will be mountedopposite the probe and nanomanipulator so that the sample to beextracted can be positioned with the illumination coming from one sideand the probe from the other. It is also preferable that theillumination source be mounted in the same plane as the probe. As aresult, rotating the sample stage so that the lamella face isperpendicular to the probe will also position the sample properlyrelative to the illumination source.

In step 704, the sample extraction probe is moved into position over thesample to be extracted. As shown in FIGS. 25-28, in order to extract thesample, the ESP probe tip 940 is oriented so that it is roughlyperpendicular to the desired probe attachment site, typically in thecenter of the sample as shown in FIGS. 27-28. As shown in FIGS. 25-26,the ESP probe tip 940 is also preferably beveled at an angle 944 ofapproximately 45 degrees, and the entire ESP probe 616 is oriented at anangle 642 of approximately 45 degree relative to the wafer upper surface(shown by dashed line 933). Where the sample to be extracted has avertical sample face, this results in the angle 932 of probe 616relative to the sample face (shown by dashed line 952) also being a 45degree angle. As a result, the beveled probe face is substantiallyparallel to the sample face. The internal walls of the probe areindicated by dashed lines 917.

To extract the sample, in step 706, the ESP probe is lowered into thecavity in front of the sample face, such as the rectangular area 924adjacent to one of the sample faces 951 as shown in FIGS. 26A-26B. Thearrow 920 in FIG. 26A indicates the direction of movement as the probeis lowered into position.

While the probe tip should be as large as possible so that the vacuumwill provide a stronger pull on the lamella, it must also be smallenough to fit into the cavity in front of the sample face to asufficient depth for the probe face to contact the side of the sample sothat the sample can be drawn to the probe tip. A suitable probe contacton a sample structure according to the present invention is shown by thedashed circle 520 in FIG. 20.

Persons of ordinary skill in the art will recognize that the internaldiameter of the probe will greatly affect air flow through the tube whena vacuum is applied. A larger internal diameter will allow for a morepowerful vacuum. However, the internal diameter will desirably besmaller than the smallest dimension of the sample to be extracted toprevent the sample from being pulled into the probe interior. In apreferred embodiment, the probe tip has a roughened face to minimizesurface contact between the sample and the probe and thus minimize anyelectrostatic or other attraction between the sample and the probe asdiscussed below.

Referring also to FIGS. 27-28, in step 708, the probe 616 is then movedtoward the sample face 951 (to the position shown by dashed line 616)and a vacuum applied through the open probe tip. The arrow 922 in FIG.27 indicates the direction of movement as the probe 616 is moved forwardto make contact with the sample. Arrow 918 shows the direction of airflow when the vacuum is applied. In step 710, once the probe tip makescontact with the sample face 951, the probe 616 can be pushed slightlyforward to break any remaining connection between the sample and thesubstrate. It may be necessary to dither the probe back and forth asshown by arrow 919 in order to completely separate the sample. Thesample is held against the probe tip by a combination of electrostaticforce and the vacuum pressure exerted through the probe tip. In someembodiments, the probe can be held in place by electrostatic forcesalone. When vacuum pressure is used to hold the sample in place, theprobe tip will preferably be adapted to minimize the electrostaticattraction between the sample and probe tip. For example, the probe tipcan have a roughened face to minimize surface contact between the sampleand the probe or it can be coated with a material that reduces theelectrostatic attraction. Minimizing the electrostatic attraction makesit easier to release the sample and to more precisely place the sampleat a desired location.

Also, in some embodiment, a probe having a conductive coating can beused to facilitate a contact sensor to determine when the probe tip isin contact with the sample. Sample contact may also be determined byusing a flow sensor to monitor pressure changes in the vacuum appliedthrough the probe tip.

In step 712, the sample can then be lifted away from the wafer until itis safely above the substrate. As shown in FIGS. 29-30, in step 714, theprobe 616 is then rotated 180 degrees, as shown by arrow 930, to placethe sample in a horizontal drop-off position. After rotation, the sampleface will lie in a plane parallel to the substrate surface. As discussedabove, the ESP probe tip 940 is preferably beveled at an angle ofapproximately 45 degrees, and the entire ESP probe 616 is oriented at anapproximately 45 degree angle to the sample face. This allows thebeveled probe tip to be substantially parallel to the vertical sampleface for sample extraction and also be substantially parallel to thehorizontal support film after rotation. Skilled persons will recognizethat the angle of the bevel and the angle of the entire probe can bevaried. However, the sum of the two angles should typically beapproximately 90 degrees where the sample has a substantially verticalface. For example, if a 60 degree bevel is used, the probe should beoriented at an angle of approximately 30 degrees (with respect to thesample face). Where angles other than 45 degrees are used, the TEM gridmay have to be tilted to allow the sample to be placed flat on the TEMgrid after the probe is rotated. In another preferred embodiment, thesample holder, such as a TEM grid, could be mounted vertically. In thatcase, rotation would not be necessary and the sample (held vertically bythe probe tip) could be placed directly on the sample holder.

In step 716, the ESP stage is then moved so that the TEM grid holder iscentered in the microscope optical field. The TEM grid holder ispreferably mounted on a rotating stage so that the grid can be alignedto the XY axis of the wafer stage by rotating the TEM grid. Preferably,the stage can also be tilted if a non-45 degree bevel/probe orientationis used. The grid can also be rotated in the appropriate direction toaccount for orientation errors in the positioning of the sample. In step718, the probe is positioned so that the sample is located above thedesired TEM grid cell. In step 720, the probe 616 is lowered until thesample 827 comes into contact with the grid support film 17 as shown inFIGS. 31-32. Line 934 indicates the direction of movement of the probe616 as the sample 827 is placed onto the support film 17. Contact can bedetermined by an appropriate contact sensor or controlled automaticallybased on known positions and calibration data. The particular film usedfor the TEM grid is preferably a film having a smooth and uniformsurface. A firmer surface is better for ensuring good surface contactbetween the lamella and the film to facilitate the transfer and accurateplacement of the lamella.

Once the sample is placed onto the TEM grid surface, in many cases thelamella will adhere to the film surface even if the vacuum through theprobe tip remains turned on. In step 721, the vacuum pressure isoptionally turned off to facilitate the release of the sample from theprobe tip. In some embodiments, in optional step 722, the vacuumpressure can be changed to a slight over-pressure 406 in order to forcethe release of a sample that remains adhered to the probe tip.

The probe is then moved to a safe distance above the TEM grid in step724 as shown in FIG. 32. Line 938 indicates the direction of movement ofthe probe after the sample has been released. If there are other samplesto be extracted in step 726, the process described in steps 702-724 isrepeated. Once all samples have been extracted, the wafer can bereturned to the production line (step 728).

The present invention provides a number of significant advantages overthe prior art. Using typical methods for TEM sample preparation, ittakes highly trained and experienced operators approximately 3 hours tocreate and extract one sample lamella suitable for TEM analysis. Forcurrent in-line metrology techniques like top-down SEM or CD-SEManalysis, as many as 20 different sites across a wafer might be need tobe measured. Using prior art methods of TEM sample preparation, it wouldtake about 60 hours just to prepare suitable TEM samples from 20different sites.

Also, because so much of the TEM sample preparation must be performedmanually, the process is very labor intensive and requires the use ofhighly skilled operators (which of course translates into high laborcosts). The increased throughput and reproducibility of the TEM analysisprovided by the present invention will allow TEM based metrology onobjects such as integrated circuits on semiconductor wafer to be usedfor in-line process control. For example, TEM analysis according to thepresent invention could be utilized in a wafer fabrication facility toprovide rapid feedback to process engineers to troubleshoot or improveprocesses. This kind of process control for the very small features thatcan only be measured by TEM is not possible using prior art TEM samplepreparation methods.

Further, current manual TEM sample preparation methods produce sampleshaving a great deal of variation. In order to use a metrology techniquefor process control, it is highly desirable that the samples be asuniform as possible. Because current methods require the final thinningof a TEM lamella to be manually controlled, there is an unavoidablevariation in sample thickness for lamellae from different sample sites.Manual control over other key elements in the sample creation process,such as fiducial placement (which determines the actual lamellalocation) introduces even more variation and further reduces theprecision of the final lamella preparation. The variation betweensamples is even greater when lamellae are prepared by differentoperators.

Using the present invention, however, results in a significantimprovement in the TEM sample preparation process. As discussed above,preferred embodiments of the present invention have been used to createand extract S/TEM samples with a thickness in the 50-100 nm range withvery minimal site-to-site variation. The process produces a lamella inroughly 18 minutes, with a site-to-site 3-sigma final lamella thicknessvariation of roughly 20 nm. The time required to sample 20 differentsites on a wafer surface drops to about 6 hours (as opposed to 60 hoursfor current methods). The process is also much less labor intensive anddoes not require operators with as high a degree of training orexperience. Because more of the process is automated, variation betweenlamella samples is also minimized.

The increased throughput and reproducibility of the TEM analysisprovided by the present invention will allow TEM based metrology onobjects such as integrated circuits on semiconductor wafer to be usedfor in-line process control. For example, TEM analysis according to thepresent invention could be utilized in a wafer fabrication facility toprovide rapid feedback to process engineers to troubleshoot or improveprocesses. This kind of process control for the very small features thatcan only be measured by TEM is not possible using prior art TEM samplepreparation methods.

The invention has broad applicability and can provide many benefits asdescribed and shown in the examples above. The embodiments will varygreatly depending upon the specific application, and not everyembodiment will provide all of the benefits and meet all of theobjectives that are achievable by the invention. For example, in apreferred embodiment TEM lamella samples are created using a galliumliquid metal ion source to produce a beam of gallium ions focused to asub-micrometer spot. Such focused ion beam systems are commerciallyavailable, for example, from FEI Company, the assignee of the presentapplication. However, even though much of the previous description isdirected toward the use of FIB milling, the milling beam used to processthe desired TEM samples could comprise, for example, an electron beam, alaser beam, or a focused or shaped ion beam, for example, from a liquidmetal ion source or a plasma ion source, or any other charged particlebeam. Also, the invention described above could be used with automaticdefect reviewing (ADR) techniques, which could identify defects viadie-to-die or cell-to-cell ADR. A lamella containing the defect could becreated and removed with or without milling fiducials. Further, althoughmuch of the previous description is directed at semiconductor wafers,the invention could be applied to any suitable substrate or surface. Thesteps described above are preferably performed automatically undercomputer control. That is, the steps are performed by the computer inaccordance with programmed instructions and without human intervention.Automatic operation does not exclude a person initiating any step or theentire process.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made to the embodiments described herein withoutdeparting from the spirit and scope of the invention as defined by theappended claims. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

We claim as follows: 1-16. (canceled)
 17. A method of processing asemiconductor wafer, comprising: selecting a process recipe includingprocess recipe values for a wafer, said process recipe includingpositional data for one or more sample sites to be analyzed; loading thewafer into a charged particle beam system, said charged particle beamsystem to process the wafer according to one or more of the selectedprocess recipe values; navigating to each sample site using saidpositional data; imaging each sample site; based on geometricinformation from CAD data describing the respective sample sites,automatically determining a precise sample location of a respectivetarget structure using image processing and metrology; automaticallypositioning and placing one or more fiducial markers at each respectivesample site; referencing the fiducial markers while automaticallymilling the wafer surface on two sides of respective sample locationsleaving a sample comprising a thin layer of material including at leasta portion of the target structure; extracting the respective samplesfrom the wafer and transferring the samples to a TEM system; imaging therespective samples with the TEM system according to one or more of theselected process recipe values; analyzing the TEM images to determine afeature dimension of the target structure.
 18. The method of claim 17wherein automatically positioning and placing one or more fiducialmarkers further comprises: identifying at least one desired firstfiducial location for the respective sample sites; milling a combinationof at least one high precision fiducial marker and one low precisionfiducial marker at fixed positions with respect to the desired firstfiducial locations; and determining edge positions for automaticallymilling the wafer surface with respect to said fiducial markers.
 19. Themethod of claim 18 in which the position of the precision fiducialmarkers is further precisely placed around the target structure usingautomated metrology calipers to find the edges of the target structure.20. The method of claim 17 further comprising adjusting the positionaldata or fiducial marker locations for at least one additional samplesite in response to the determined dimension.
 21. The method of claim 17further comprising providing CAD data to specify the location of the oneor more fiducial markers with respect to the target structure on thewafer.
 22. The method of claim 17 further comprising, after imaging thesamples with TEM, returning the wafer to a production line.
 23. Themethod of claim 17 further comprising, based on the measured featuredimensions, adjusting a fabrication process on a production line toobtain a desired feature dimension for similar features.
 24. The methodof claim 17 wherein automatically milling the substrate surface on twosides of a desired sample comprises automatically milling the substratesurface on either side of a desired sample location leaving a thin layerof material less than 100 nm thick.
 25. The method of claim 17 whereinextracting the samples from the wafer comprises automatically extractingthe samples from the wafer.
 26. A method of processing a semiconductorwafer, comprising: selecting a process recipe including process recipevalues for a wafer, said process recipe including positional data forone or more sample sites to be analyzed; loading the wafer into acharged particle beam system, said charged particle beam system toprocess the wafer according to one or more of the selected processrecipe values; navigating to each sample site using said positionaldata; imaging each sample site; automatically determining a precisesample location of a respective target structure using image processingand metrology; providing CAD data to specify the location of the one ormore fiducial markers with respect to the target structure on the wafer,automatically positioning and placing one or more fiducial markers ateach respective sample site; referencing the fiducial markers whileautomatically milling the wafer surface on two sides of respectivesample locations leaving a sample comprising a thin layer of materialincluding at least a portion of the target structure; extracting therespective samples from the wafer and transferring the samples to a TEMsystem; imaging the respective samples with the TEM system according toone or more of the selected process recipe values; analyzing the TEMimages to determine a feature dimension of the target structure.
 27. Themethod of claim 26 wherein automatically positioning and placing one ormore fiducial markers further comprises: identifying at least onedesired first fiducial location for the respective sample sites; millinga combination of at least one high precision fiducial marker and one lowprecision fiducial marker at fixed positions with respect to the desiredfirst fiducial locations; and determining edge positions forautomatically milling the wafer surface with respect to said fiducialmarkers.
 28. The method of claim 27 in which the position of theprecision fiducial markers is further precisely placed around the targetstructure using automated metrology calipers to find the edges of thetarget structure.
 29. The method of claim 26 further comprisingadjusting the positional data or fiducial marker locations for at leastone additional sample site in response to the determined dimension. 30.The method of claim 26, automatically determining a precise samplelocation of a respective target structure using image processing andmetrology is based on geometric information from CAD data describing therespective sample sites.
 31. The method of claim 26 further comprising,after imaging the samples with TEM, returning the wafer to a productionline for further processing.
 32. The method of claim 26 furthercomprising, based on the measured feature dimensions, adjusting afabrication process on a production line to obtain a desired featuredimension for similar features.
 33. The method of claim 26 whereinautomatically milling the substrate surface on two sides of a desiredsample comprises automatically milling the substrate surface on eitherside of a desired sample location leaving a thin layer of material lessthan 100 nm thick.
 34. The method of claim 26 wherein extracting thesamples from the wafer comprises automatically extracting the samplesfrom the wafer.